Electrical resistance heaters formed of helically wound resistance wire are widely employed in high temperature furnaces. Such heaters are typically supported by ceramic cores such as grooved plates or cylinders in which the heater is supported and confined throughout its entire length by the ceramic structure. The ceramic core usually has a plurality of longitudinal grooves formed therein and surrounding the coil around a major portion of its periphery. The groove may be filled with refractory sealing material such that the heater is fully embedded in a ceramic core, or a packed ceramic powder can surround the heater coil and be sheathed by a metal tube.
The weight of the ceramic support structure constitutes a major percentage of the overall heater assembly mass by reason of the of ceramic necessary for support of the heater coil and the inherent density of the ceramic material. Such ceramic support structures have relatively low thermal insulation properties and as a result of the relatively massive amount of ceramic material present, a heater of conventional construction exhibits a high thermal inertia which limits the rapidity with which a change of temperature can be accomplished. The response of such conventional heaters to temperature control is thereby limited by the relatively slow thermal response of the heater structure. The high thermal inertia also affects the overall efficiency of a conventional heater since the heat must saturate the surrounding ceramic material before direct radiation to the product from the heater can significantly occur. The ceramic core even in those conventional heaters having an open groove effectively shades all or a major portion of the direct radiation emitted by the heater coil thereby providing a low emissivity, which in turn promotes a substantial differential in temperature between the product and the heating coil, causing inefficiency and shorter heater life.
In heaters of conventional construction supported by a ceramic structure, the length, weight and mode of mounting the heater is dictated to a great extent by the strength of the ceramic support. Self-supporting heaters are known in which ceramic supporting structures are not employed but such conventional self-supporting heaters suffer other deficiencies. A helical silicon carbide heater is shown in U.S. Pat. No. 3,859,501 in which the heating element is in the form of a helical silicon carbide element having a small gap between the helical turns. A voltage differential exists across the gap and can be of sufficient magnitude to cause a voltage discharge especially in the presence of contaminants which condense or otherwise become disposed in the gap. The helical heater construction is also structurally weak.
Another conventional refractory heater known as a Norton SU heater employs two parallel rods of silicon carbide each having a high resistance portion and a low resistance portion. The high resistance portions of the rods are, in operation, disposed within a furnace chamber and are connected at their ends by a connecting block of silicon carbide. The connecting block is of a size and configuration to require a relatively large opening in the furnace wall or roof for insertion of the heater into the chamber. In addition, an insulative two hole plug must be precisely mated to the heater to retain the parallel rods within the mounting opening in the furnace. The parallel rod construction is also subject to the deleterious effects of unequal bending stresses during furnace operations. Moreover, the connecting block is of substantial mass such that if the heater is suspended from the roof of a furnace chamber, the heater can be subject to pendulous movement which can cause bending stresses and cracking of the heater rods.
A coaxial heater construction is shown in U.S. Pat. No. 3,764,718 which is of specific design for use in a vacuum furnace. This heater includes a tubular resistor element and a coaxially disposed inner resistor element connected at one end to the surrounding tube. The inner and outer elements are in a primary embodiment of the same resistive material which is stated to be carbon, silicon carbide, metal or metal alloy, and both inner and outer elements serve as heaters, the inner element radiating heat through the outer element which also radiates heat from its surface. In another version of this heater, the outer element is operated primarily as a conductor and as a radiant element for heat generated by one or more inner resistor elements. The heater is movably mounted above a vacuum chamber and the outer element is adapted to be disposed within the chamber for radiation from its entire surface.
An improved silicon carbide heater is shown in U.S. Pat. No. 4,080,510, assigned to the assignee of this invention, which comprises an elongated tubular high temperature element having first and second tubular sections contiguous with one another, the first section being of a high resistivity silicon carbide and adapted for disposition within a furnace chamber, the second section being of a low resistivity silicon carbide and adapted for disposition external to the furnace chamber. An elongated rod of silicon carbide of low resistivity material is disposed coaxially within the tubular element and is in electrical connection with an end of the first tubular section. The coaxial ends of the second tubular section and of the rod include contact areas for electrical connection to an external power source. The high resistivity tubular section provides efficient heating, while the low resistivity tubular section and coaxial rod provide a conductive electrical path to the heating section while minimizing the heat thereof.